Most comfortable way, flying: Hannover has an airport. From Dublin as far as I know there are no direct flights, but with stop in Amsterdam. Prices from 340€ on (Opodo), flight time depends on the waiting time in Amsterdam, but in best case 3:20 h. A cheaper alternative is to fly from Dublin to Hamburg (100km away from Hannover, one stop in Frankfurt). Price less than 200 € for the two flights (link to the flight, if you stay the weekend) +42 € for train from Hamburg to Hannover (Niedersachsenticket, 21 € for each trip), longer waiting time at Frankfurt (total flight time 5h + ~1h train). Cheaper methods are probably only possible, if you hitchhike or if some fanclubs offer organised trips. Anyways, even if I would be a bit envious, if Hannover play you. I wish St.Pat's all the best in the current round and a nice party vs. Hannover.